Hydrophilic or hydrophobic coating of whey protein aerogels obtained by supercritical-CO₂-drying: Effect on physical properties, moisture adsorption and interaction with water and oil in food systems

Aerogel monoliths, prepared by water-to-ethanol substitution and supercritical-CO2-drying of whey protein hydrogels, were dip-coated with hydrophilic (alginate, AL; agar, AG) or hydrophobic (ethylcellulose, EC) polymers. AL coating induced aerogel collapse, due to solvent absorption. AG and EC rapidly set onto aerogel surface, forming layers of 65 and 100 mu m thickness, respectively. While AG-coating induced 20% volume shrinkage, 25% apparent density increase and 75% firmness increase, EC-coating maintained the original aerogel structure. Upon exposure to 100% equilibrium relative humidity, aerogels showed moisture uptake in the order AG-coated>uncoated>EC-coated. When immersed in water or oil, the AG-coated aerogel showed an uptake respectively 40 and 60% lower than the uncoated control. The oil barrier capacity of AG-coated aerogel was also demonstrated in a lipid food system (stearin-oil mixture). Although not reducing oil uptake, EC-coating reduced water uptake by 30% and its water barrier properties were demonstrated upon immersion in an aqueous food system (water-flour batter).